In a communication network such as a wireless communication or a PLC (Power Line Communications), in order to prevent information leakage to the outside of the network, a commonly used method is to perform an authentication process for sharing an encryption key to be used in communication between a communication device which currently participates in the network and a communication device which tries to newly participate in the network, and once the authentication process is terminated, to conduct communication by encrypting data using the shared encryption key.
In one example of the authentication process, a user sets a common authentication key both in a slave unit which requests for authentication and a master unit which authenticates the slave unit, and the master unit, when it has confirmed that the slave unit has the same authentication key as that the master unit has, distributes an encryption key to the slave unit. However, unless the slave unit has a function to receive an input of an authentication key from outside, the authentication key that the slave unit retains cannot be changed from the one set at the time of manufacture of the slave unit. Accordingly, the security may not be secured.
In order to solve such a problem, there are, for example, technologies which are disclosed in Patent literatures 1 and 2. According to the technology disclosed in Patent literature 1, by pressing down both of buttons respectively provided in a master unit and a slave unit within a predetermined time period, an authentication is conducted only between the master unit and the slave unit whose buttons are pressed down within the predetermined time period. However, according to the technology disclosed in Patent literature 1, when a plurality of slave units are authenticated at one time, the user needs to repeat, with respect each of the slave units, an operation in which the user presses down buttons provided in the master unit and each of the slave unit within the predetermined time period and waits for an authentication to be terminated. A procedure like this imposes a heavy burden on the user, and thus needs to be reduced.
According to the technology disclosed in Patent literature 2, information specific to a client such as an ID and a password is preset in a server, and at the time of authentication, the client encrypts the information specific to the client by using a public key distributed by the server and transmits the encrypted information to the server. Then, when the information received by the server is identical with the preset information, the client is authenticated. However, the technology disclosed in Patent document 2 has a problem that, a Man in the Middle is present between a client and a server in a network and when the Man in the Middle manipulates a public key of the server, security cannot be secured. Further, when a public encryption key is used, a time required for encrypting and decrypting is longer than that required in a common key encryption method. Accordingly, when a plurality of slave units having low-end CPUs are to be authenticated, a time required for authenticating all the slave units becomes long.